The ability of T cells to mediate an immune response against an antigen requires two distinct signaling interactions (Viglietta, V. et al. (2007) Neurotherapeutics 4:666-675; Korman, A. J. et al. (2007) Adv. Immunol. 90:297-339). First, an antigen that has been arrayed on the surface of antigen-presenting cells (APC) is presented to an antigen-specific naive CD4+ T cell. Such presentation delivers a signal via the T cell receptor (TCR) that directs the T cell to initiate an immune response specific to the presented antigen. Second, various co-stimulatory and inhibitory signals mediated through interactions between the APC and distinct T cell surface molecules trigger the activation and proliferation of the T cells and ultimately their inhibition.
The immune system is tightly controlled by a network of costimulatory and co-inhibitory ligands and receptors. These molecules provide the second signal for T cell activation and provide a balanced network of positive and negative signals to maximize immune responses against infection, while limiting immunity to self (Wang, L. et al. (Epub Mar. 7, 2011) J. Exp. Med. 208(3):577-92; Lepenies, B. et al. (2008) Endocrine, Metabolic & Immune Disorders—Drug Targets 8:279-288). Examples of costimulatory signals include the binding between the B7.1 (CD80) and B7.2 (CD86) ligands of the APC and the CD28 and CTLA-4 receptors of the CD4+ T-lymphocyte (Sharpe, A. H. et al. (2002) Nature Rev. Immunol. 2:116-126; Lindley, P. S. et al. (2009) Immunol. Rev. 229:307-321). Binding of B7.1 or B7.2 to CD28 stimulates T cell activation, whereas binding of B7.1 or B7.2 to CTLA-4 inhibits such activation (Dong, C. et al. (2003) Immunolog. Res. 28(1):39-48; Greenwald, R. J. et al. (2005) Ann. Rev. Immunol. 23:515-548). CD28 is constitutively expressed on the surface of T cells (Gross, J., et al. (1992) J. Immunol. 149:380-388), whereas CTLA4 expression is rapidly up-regulated following T-cell activation (Linsley, P. et al. (1996) Immunity 4:535-543).
Other ligands of the CD28 receptor include a group of related B7 molecules, also known as the “B7 Superfamily” (Coyle, A. J. et al. (2001) Nature Immunol. 2(3):203-209; Sharpe, A. H. et al. (2002) Nature Rev. Immunol. 2:116-126; Collins, M. et al. (2005) Genome Biol. 6:223.1-223.7; Korman, A. J. et al. (2007) Adv. Immunol. 90:297-339). Several members of the B7 Superfamily are known, including B7.1 (CD80), B7.2 (CD86), the inducible co-stimulator ligand (ICOS-L), the programmed death-1 ligand (PD-L1; B7-H1), the programmed death-2 ligand (PD-L2; B7-DC), B7-H3, B7-H4 and B7-H6 (Collins, M. et al. (2005) Genome Biol. 6:223.1-223.7).
The Programmed Death 1 (PD-1) protein is an inhibitory member of the extended CD28/CTLA4 family of T cell regulators (Okazaki et al. (2002) Curr Opin Immunol 14: 391779-82; Bennett et al. (2003) J. Immunol. 170:711-8). Other members of the CD28 family include CD28, CTLA-4, ICOS and BTLA. PD-1 is suggested to exist as a monomer, lacking the unpaired cysteine residue characteristic of other CD28 family members. PD-1 is expressed on activated B cells, T cells, and monocytes.
The PD-1 gene encodes a 55 kDa type I transmembrane protein (Agata et al. (1996) Int Immunol. 8:765-72). Although structurally similar to CTLA-4, PD-1 lacks the MYPPY motif (SEQ ID NO: 1) that is important for B7-1 and B7-2 binding. Two ligands for PD-1 have been identified, PD-L1 (B7-H1) and PD-L2 (B7-DC), that have been shown to downregulate T cell activation upon binding to PD-1 (Freeman et al. (2000) J. Exp. Med. 192:1027-34; Carter et al. (2002) Eur. J. Immunol. 32:634-43). Both PD-L1 and PD-L2 are B7 homologs that bind to PD-1, but do not bind to other CD28 family members. PD-L1 is abundant in a variety of human cancers (Dong et al. (2002) Nat. Med. 8:787-9).
PD-1 is known as an immunoinhibitory protein that negatively regulates TCR signals (Ishida, Y. et al. (1992) EMBO J. 11:3887-3895; Blank, C. et al. (Epub 2006 Dec. 29) Immunol. Immunother. 56(5):739-745). The interaction between PD-1 and PD-L1 can act as an immune checkpoint, which can lead to, e.g., a decrease in tumor infiltrating lymphocytes, a decrease in T-cell receptor mediated proliferation, and/or immune evasion by cancerous cells (Dong et al. (2003) J. Mol. Med. 81:281-7; Blank et al. (2005) Cancer Immunol. Immunother. 54:307-314; Konishi et al. (2004) Clin. Cancer Res. 10:5094-100). Immune suppression can be reversed by inhibiting the local interaction of PD-1 with PD-L1 or PD-L2; the effect is additive when the interaction of PD-1 with PD-L2 is blocked as well (Iwai et al. (2002) Proc. Nat'l. Acad. Sci. USA 99:12293-7; Brown et al. (2003) J. Immunol. 170:1257-66).
The epidermal growth factor receptor (EGFR, Erb-B1) belongs to a family of proteins involved in the proliferation of normal and malignant cells. Overexpression of EGFR is found in over 70 percent of human cancers, including without limitation non-small cell lung carcinomas (NSCLC), breast cancers, gliomas, squamous cell carcinoma of the head and neck, and prostate cancer. The identification of EGFR as an oncogene has led to the development of EGFR tyrosine kinase inhibitors, such as gefitinib, erlotinib, or afatinib, and their deployment in a treatment of cancers, in particular non-small cell lung cancer, harboring EGFR mutations. However, almost all patients eventually develop resistance to the treatment with such EGFR tyrosine kinase inhibitors.
Given the importance of immune checkpoint pathways in regulating an immune response and overexpression of EGFR in a number of cancers, the need exists for developing novel combination therapies that activate the immune system.